This application discloses an invention that is related, generally and in various embodiments, to a transformer with reduced size, safety insulation and low leakage inductance. Power supplies are becoming more compact to keep pace with the reduction in size of the equipment they provide energy for. The power processing components, including the main transformer, comprise a significant portion of the power supply's volume, and the heat the components dissipate requires bulky cooling apparatus such as heatsinks and fans. By reducing the power loss of the power supply, the size of the cooling apparatus can be reduced, and components like the main transformer are a key target for reducing the power loss of the power supply.
Power supply transformers provide the isolation barrier between the high AC input voltage and the low DC output voltages, which may be contacted by personnel, so safety rated insulation is required in each transformer. To meet the mandates of various safety agencies, a power transformer must include either defined spacings between conductors or have multiple layers of insulation separating the conductors. These spacing requirements add to the size of power transformers and increase leakage inductance, which interferes with the rapid transfer of energy from the primary to the secondary of the transformer. For example,
Operating at a high frequency is a desirable way to reduce transformer size. However, leakage inductance is particularly problematic as switching frequency increases. Although dividing the windings into multiple sections and interleaving the primary and the secondary reduces leakage inductance, the multiple sections require additional layers of insulation and make manufacture of such transformers difficult and expensive. In addition, because significant power transfer at a low output voltage leads to a high output current, the secondary winding cross-section must be relatively large.
A well-known solution for some of these problems is the planar transformer, which employs multi-layer printed circuit technology to create an interleaved winding structure. However, as the technology is limited to thin copper sheet, many layers are needed to achieve significant conductor cross-section. Accordingly, planar transformers often have 12, 16 or more layers of conductor and associated insulating laminate. Unfortunately, the multiple layers of insulation block heat transfer, reduce volumetric efficiency and increase cost.